First discovered in the late 1940's, holography is a method of optical imagery similar in some respects to photography. With the advent of the laser as a coherent light source, a wide variety of practical uses for holography have been developed since the early 1960's.
While similar in some respects, photography and holography are fundamentally different. Unlike photography, which produces a two-dimensional image of an object by recording the irradiance distribution of light reflected by the object, holography produces a three-dimensional image of an object by recording a wavefront of light emanating from the object.
Typically, this object wave is recorded in such a way that subsequent illumination of the record can reconstruct the original object wave. Visual observation of the reconstructed wavefront then yields a three-dimensional view of the object that is virtually indistinguishable from the original.
A hologram is generally produced from an expanded coherent light beam that is split into two components. One component is directed toward an object and is referred to as the illuminating, or object beam. The other component is directed toward a recording medium and is referred to as the coding, or reference beam. Typically, the object beam is reflected off the object toward the recording medium. Since the object and reference beams originate from the same source, they are mutually coherent and their interference forms a stable pattern when they meet at the recording medium. The record of the interference pattern of the object and reference beams constitutes a hologram.
Conventional holographic imaging systems have been modified over the years in a number of different fashions. For example, an object may be illuminated from behind by the object beam. Such a technique produces a holographic "shadowgram" and was disclosed in "Wavefront Reconstruction with Diffused Illumination and Three-Dimensional Objects" by E. Leith and J. Upatnieks in the Journal of the Optical Society of America, Vol. 54, No. 11, Nov. 1964. Moreover, single beam holographic imaging systems have also been developed, such as those disclosed in "Single Beam Holography" by H. Chen and P. Ruterbusch in the American Journal of Physics, Vol. 47, No. 12, Dec. 1979.
The typical holographic imaging systems described above, however, suffer from various problems. The most troublesome of such problems is that the object being recorded must remain perfectly still to within a fraction of a micrometer and must be devoid of any minute vibration during the period of recording. More specifically, when the coherent light beam is split in conventional holographic imaging systems, there is a potential for undesirable differential fringe vibration of the interference pattern between the resultant object and reference beams. Any motion or vibration in the object or optical component creates a differential vibration, or optical path difference, between the object beam and the reference beam. Such a change in the optical path difference of the object and reference beams destabilizes the interference pattern between the beams thereby undermining the effectiveness of the resultant hologram. These stringent motion and vibration requirements have thus ruled out the use of human or other live specimen as the holographic object in conventional holographic imaging systems.
The stringent motion and vibration requirements of conventional holographic imaging systems also create a variety of secondary problems. To reduce vibration problems, expensive precision components are required, including large vibration isolated optical tables. The motion and vibration problems of typical holographic imaging systems can be reduced through the use of single beam techniques, or virtually eliminated through the use of pulse lasers. However, known single beam techniques cannot easily and sufficiently eliminate motion and vibration problems, and pulse lasers significantly increase the cost of holographic imaging systems. Moreover, with either conventional or pulse laser holographic imaging systems, the holographic recording process is a sophisticated technique that can only be carried out by skilled technicians in a well equipped laboratory.
The conventional and pulse laser holographic imaging systems described above, and their various applications, are exemplified by U.S. Pat. Nos. 3,692,381 to Champagne; 4,278,319 to Jozsa et al; 4,566,757 to Fusek et al; 4,603,937 to Copp; and 5,177,296 to Hoebing. Each of these patents, however, disclose holographic imaging systems that suffer from the problems discussed above, such as differential vibration between the object and reference beams, or significant expense.